Doctor Who Season 10: 7 Big Questions After 'Extremis'
4. Why Did The TARDIS Translation Circuit Not Work?
In the Ninth Doctor’s second adventure, The End of the
World, the Doctor explained the age old question of why our heroes are almost
always able to understand foreign tongues, whether human or alien. It’s all
down to the TARDIS’s translation circuit. It was a slightly more plausible
explanation to the one the Fourth Doctor gave Sarah in The Masque of Mandragora
when he said it was a Time Lord gift he shared.
In The Christmas Invasion the two alternative explanations come together in the notion that the circuit can only function correctly with the Doctor present. So if both the TARDIS and the Doctor are in Bill’s room, then why does the Pope speak in Italian?
In an interview with the Radio Times, Moffat has revealed that a line of dialogue that explained the TARDIS's failure to translate was cut. Nardole encouraged the Doctor to allow the Pope to be heard in his native tongue out of respect. Moffat goes on to suggest that a better solution might be that the circuit is faulty on account of the Doctor’s blindness. Quite why this would make any difference isn't immediately obvious, with the opposite more likely to occur as the other senses compensate for the loss of sight.
The circuit hasn't exactly been completely reliable over the years, and so there really is no need to explain it as anything other than a temporary glitch. But fans looking for an explanation internal to the plot could simply put it down to the fact that this whole scene and its players (including the TARDIS) are part of the simulation.